1. Field of the Invention
The present invention relates to an anti-reflection layer for semiconductor integrated circuit. More particularly, the present invention relates to a silicon oxynitride (SiO2xNy) anti-reflection layer and its method of manufacture.
2. Description of the Related Art
The integrated circuit patterns of a semiconductor chip are formed by transferring the desired pattern from a photomask to a photoresist layer through light exposure and subsequent photoresist development. In general, light that passes through the unblocked regions of a photomask will cause a photochemical reaction in the photoresist material. The reacted photoresist material is next removed in a photoresist development to reproduce the photomask pattern on the photoresist layer. However, due to variations in local topography of a silicon chip, the photoresist layer is unlikely to have a uniform thickness or uniform reflectivity to light. When the photoresist layer is exposed to light, incoming light may interfere with light reflected from the photoresist/substrate interface. Consequently, what are known as reflective notches are often formed in some regions of the transferred pattern, leading to errors in device line width or feature misalignment.
To reduce device line width, light source of very short wavelength is now used in new generation optical exposure systems. However, a shorter wavelength light beam has greater reflectivity at the photoresist/chip interface, causing larger deviations in the resulting photoresist pattern. In addition, as intensity level of back-reflected light is increased, accuracy of the pattern is harder to control. Such instability of resulting pattern is easily observed when a polysilicon gate or a layer with a highly reflective underlying medium such as aluminum is patterned. Hence, constrained by small line width and high reflectivity, high fidelity pattern transfer in a photolithographic operation proves difficult to obtain.
Accordingly, one object of the present invention is to provide an anti-reflection layer capable of reducing the intensity of reflected light from a reflective surface. Hence, errors in transferred pattern due to interference of incoming and reflected light are minimized. In addition, a wider window of depth of focus (DOF) for a photoresist layer can be obtained so that precision and resolution of a pattern transfer is raised.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an anti-reflection layer having a composite structure. The composite anti-reflection structure includes an oxygen-rich silicon oxynitride layer, a nitrogen-rich silicon oxynitride layer and a stack of silicon oxynitride layers. Oxygen content in each silicon oxynitride layer is lower than the oxygen-rich silicon oxynitride layer while nitrogen content of each silicon oxynitride layer is lower than the nitrogen-rich oxynitride layer. In addition, among the stack of silicon oxynitride layers, the layer closest to the oxygen-rich silicon oxynitride layer has the highest oxygen content. Similarly, among the stacks of silicon oxynitride layers, the layer closest to the nitrogen-rich silicon oxynitride layer has the highest nitrogen content.
The invention also provides a method of forming the anti-reflection layer. The method includes depositing silicon oxynitride over a reflective surface in a reacting chamber using silane, ammonia and nitrous oxide as reactive gases in a plasma-enhanced chemical vapor deposition. A constant flow of silane and ammonia is maintained in the reaction chamber throughout the reaction. Flow rate of nitrous oxide is varied such that the flow rate drops from 2 liters per minute at the beginning of the reaction to almost zero near the end of the reaction. Hence, oxygen content in the deposited silicon oxynitride layer will drop while nitrogen content in the deposited silicon oxynitride layer will increase as the reaction progresses. In other words, while an oxygen-rich silicon oxynitride layer is formed at the bottom while a nitrogen-rich silicon oxynitride layer is formed at the top such that oxygen and nitrogen content of the intermediate layers changes gradually.
The silicon oxynitride structure in the embodiment of this invention is oxygen-rich at the bottom and nitrogen-rich at the top. Between the oxygen-rich bottom layer and the nitrogen-rich top layer is silicon oxynitride material having a gradually varying amount of oxygen and nitrogen. Hence, the silicon oxynitride structure can be regarded as a stack of silicon oxynitride layers, each having a different oxygen/nitrogen ratio and hence a slightly different physical property. The stack of silicon oxynitride layers can prevent back reflection of light because light will be reflected and refracted differently by each layer. After several reflections and refractions, intensity of back-reflected light is reduced to a negligible level. Furthermore, through multiple reflections and refractions, the depth of focus window is increased so that the ultimate resolution of the photoresist layer is also increased.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.